It is known to use an electronic circuit to determine the depth of a liquid. Below follows a brief summary of the prior art.
U.S. Pat. No. 4,122,718 (1978) to Gustafason discloses an AC circuit that measures capacities along a tube (FIG. 7 and claim 13-16) filled with a liquid in a tank.
U.S. Pat. No. 4,354,180 (1982) to Harding discloses a light beam based low level detector.
U.S. Pat. No. 4,449,384 (1984) to Jones discloses an appliance controller.
U.S. Pat. No. 4,459,584 (1984) to Clarkson discloses a magnetic sensor.
U.S. Pat. No. 4,814,752 (1989) to Lehman discloses a float switch.
U.S. Pat. No. 4,949,069 (1990) to Wilson discloses liquid sensing probe connected to a time delay low level alarm.
U.S. Pat. No. 4,982,606 (1991) to Adamski et al. discloses a capacitance based level detector using plates immersed in the tank.
U.S. Pat. No. 5,026,954 (1991) to Cebulski discloses a float switch.
U.S. Pat. No. 5,028,910 (1991) to Meacham et al. discloses a float switch.
U.S. Pat. No. 5,210,769 (1993) to Seidel et al. discloses a level sensor using a DC based positive temperature coefficient conductor. See col. 4 lines 5-38 for details of the “Balco” wire which changes resistance based on its surrounding fluid level. See also col. 9 lines 40 et seq.
U.S. Pat. No. 5,493,877 (1996) to Wickremasinghe discloses a fluid presence sensor using a plastic covered foil.
U.S. Pat. No. 5,565,687 (1996) to Berrill discloses Hall effect switches to detect a floating magnet.
U.S. Pat. No. 5,602,540 (1997) to Spillman Jr. discloses an AC oscillator circuit to detect liquid level. Gustafson should have been cited by the Examiner.
U.S. Pat. No. 5,613,399 (1997) to Hannan et al. discloses a plurality of capacitance plates stacked inside a container with a controller to calculate depth of immersion.
U.S. Pat. No. 5,743,135 (1998) to Sayka et al. discloses an optical float switch.
U.S. Pat. No. 5,773,913 (1998) to Casselden discloses a piezoelectric sensor.
U.S. Pat. No. 6,028,521 (2000) and U.S. Pat. No. 6,218,949 to Issachar discloses a magnet float switch and a waveguide level detector.
U.S. Pat. No. 6,624,755 (2003) to Chamberlin discloses a resistive probe and a ring to prevent a false reading.
U.S. Pat. No. 6,972,685 (2005) Sato et al. discloses a float switch.
None of the above noted inventions have made commercial inroads into measuring the depth of the liquid in an oil/gas well. The present invention has been successfully tested in an oil/gas well application.